A lot of people on here will tell you that an x-mart bike will break down, not work, or possibly hurt you. Everyone else will say that they've ridden one, loved it, gone hundreds of thousands of miles on one, and never had a problem.
Here is my opinion and advice, having both owned, ridden and worked on later x-mart bikes.
I never had a problem with the Huffy I had. It wasn't the greatest bike out there, but it got me around when i was in high school. The x-mart bikes i've worked on in the last few years have been a little more problematic with things not being set up correctly or adjusted.
The Good:
X-mart bikes are inexpensive. They're new. If something goes wrong or you change your mind, you could most likely return/exchange it, like any other merchandise from the store (check on this, i'm only assuming).
The Bad:
X-mart bikes tend to be lower quality, using the cheapest parts made. These bikes also tend to be a lot heavier, compared to higher end bikes. Suspension systems employed by x-mart bikes tend not to be worth the extra money, as they add weight and create inefficiency in the drivetrain. Most importantly, the x-mart bikes are assembled by x-mart employees (stockboys, etc.) and not bicycle mechanics, which can lead to things being put together incorrectly (brakes not set right, deraileurs not set right, forks put on backwards, etc.).
My recommendation:
If you get an x-mart bike, go over every inch of it before you ride it. Make sure it was assembled correctly and that everything is adjusted correctly. Don't trust that they put everything together right. If you can repair/fix bikes, awesome. Otherwise have a friend who does look it over. Bringing it to a bike shop will become costly. I would also recommend the non-suspension bikes, or front suspension only. Fewer things to break, probably a little lighter also.
If you do know how to fix/wrench on bikes, i would recommend going second hand. Even if you want to learn, you can find an older bicycle that you like for very little money (check thrift stores, garage sales, etc.), and learn.
My best advice, above all, is to find a bike you love. If you love the bike, you'll ride it, no matter what it is. If you get a bike, and you don't like it, you're probably not as likely to go ride.
That's my two cents. Let me know what you decide.